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What is the grammar rule for the preposition after the word free? Why do we say free of impurities but free of charge?

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Welcome to English SE, @teacher! I'm not sure what you're asking here; why "impurities" is plural but "charge" is not, when following "free of"? – Pops Feb 4 '11 at 15:28

The "charge" vs. "impurities" difference you actually seem to be asking about is nothing to do with the preposition as such; it's to do with "charge" being a "mass noun", so not calling for pluralization, and "impurity" being a "count noun", which it's appropriate to pluralize here.

This distinction is the same one as produces the "less vs. fewer" dichotomy.

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There are other prepositions you could use with "free" as well:

  • I am free from two till four this afternoon.
  • I would like to be free of this burden.
  • A soft drink comes free with purchase.
  • You are free to do what you want.
  • The offer is free while supplies last.
  • Hang on, I'll be free in five minutes.
  • I'm free at last!

I could go on ...

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